Sewer chimney systems are used to surround and protect the generally vertical chimney pipe which rises from a "wye" or "tee" connection at the generally horizontal main sewer line and interconnects at another "wye" or "tee" with the generally horizontal service pipe from a home or business building. One type of chimney system in current use is comprised of a circular cardboard form which is placed around the chimney pipe into which fresh concrete is poured. The fresh concrete is also placed around the "wye" or "tee" below, thereby creating a solid mass of concrete bearing directly on the sewer main. Concrete weighs about 150 pounds per cubic foot as compared to sand or gravel, which weighs about 100 pounds per cubic foot. Most leaks are found to occur at the chimney locations due to differential settling caused by the differential loading.
Fresh concrete, of course, is obtained by calling a concrete mixing plant to deliver the concrete by truck, or the concrete is mixed on the job site with a portable mixer, or by hand. In either case the process is very time consuming and expensive. Another disadvantage is the fact that no backfilling of earth is permitted around this chimney system until the fresh concrete has hardened, which usually means at least the next day, thus also causing a delay in progress.
A second sewer chimney system currently in use consists of a chimney pipe cast within a precast concrete section. The system consists of a precast base section, precast riser section of two, three or four foot height, and a precast top section which consists of the "wye" or "tee" which receives the sewer service pipe. The sections are stacked on each other and bolted together to prevent leakage and toppling.
Once installed, the system can be backfilled with earth. There are several problems associated with such a system. Several pieces of heavy construction equipment are used for the process. On delivery to the job site a front-end loader with operator and a laborer is needed to unload the concrete sections and deposit them at a storage site. The concrete sections are once again picked up at a later date with a front end loader with an operator, a laborer, and possibly a truck and driver for delivery to the work site. The backhoe, which is generally used to excavate the sewer trench, is then used to lift the concrete sections and place them into position over the "wye" or "tee". The sewer main installation is slowed down considerably because the backhoe which should be digging is now installing the precast chimney. A crew of six to eight men plus the equipment is virtually brought to a halt during this phase.
Leaks are a significant problem with this system due to the integration of the chimney pipe within the concrete sections.
The base portion of this precast system, which supports the riser sections and top section, straddles the sewer main. However, this system is capable of causing failure to the sewer main if settlement of the precast concrete sections occurs. Even though the base section straddles the sewer main and is initially set a few inches above the top of the sewer main, settlement due to the excessive weight can cause forces to be exerted on the "wye" or "tee" which is making direct contact with it.